From: Matthew Wilcox <willy@infradead.org>
To: Aswin Kumar <aswinkumar3301@gmail.com>
Cc: linux-mm@kvack.org, andrew.morton@linux-foundation.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/userfaultfd: fix likely/unlikely annotation in move_pages()
Date: Wed, 4 Feb 2026 14:58:34 +0000 [thread overview]
Message-ID: <aYNemlPABKnQkf-x@casper.infradead.org> (raw)
In-Reply-To: <20260204142350.1642-1-aswinkumar3301@gmail.com>
On Wed, Feb 04, 2026 at 02:23:50PM +0000, Aswin Kumar wrote:
> All other userfaultfd paths use unlikely() for the mmap_changing check,
No they don't?
$ git grep mmap_changing mm
mm/userfaultfd.c: if (atomic_read(&ctx->mmap_changing))
mm/userfaultfd.c: if (atomic_read(&ctx->mmap_changing))
mm/userfaultfd.c: if (atomic_read(&ctx->mmap_changing))
mm/userfaultfd.c: if (likely(atomic_read(&ctx->mmap_changing)))
> This is a performance-only fix - the logic is correct but the branch
> prediction annotation is wrong, potentially causing a minor performance
> penalty on the fast path.
Can you measure it? Bet you can't.
Honestly, I'd just remove the likely() annotation, not change it to
unlikely().
next prev parent reply other threads:[~2026-02-04 14:58 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-02-04 14:23 Aswin Kumar
2026-02-04 14:58 ` Matthew Wilcox [this message]
2026-02-12 12:47 ` David Hildenbrand (Arm)
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